This invention relates to a sterilization process in which a small amount of sterilant is introduced into the chamber, the concentration of sterilant in the chamber is measured to determine the load, and more sterilant is introduced, if necessary, based on the determined load.
Medical instruments have traditionally been sterilized using heat, for example steam, or a chemical, such as formaldehyde or ethylene oxide in the gas or vapor state. Both heat and chemical sterilization have drawbacks. For example, many medical devices, such as fiberoptic devices, endoscopes, power tools, etc. are sensitive to heat, moisture, or both.
Formaldehyde and ethylene oxide are both toxic gases which pose a potential hazard to health workers. After sterilization with ethylene oxide, the articles require long aeration times to remove the gas from articles which have been sterilized. This makes the sterilization cycle time undesirably long.
Sterilization using liquid hydrogen peroxide solutions has been found to require a high concentration of sterilant, extended exposure times and/or elevated temperatures. However, sterilization using hydrogen peroxide vapor has been shown to have some advantages over other chemical sterilization processes (see, for example, U.S. Pat. Nos. 4,169,123 and 4,169,124). The combination of hydrogen peroxide vapor and a plasma provides additional advantages, as disclosed in U.S. Pat. No. 4,643,876. U.S. Pat. No. 4,756,882 discloses the use of hydrogen peroxide vapor, generated from an aqueous solution of hydrogen peroxide, as a precursor of the reactive species generated by a plasma generator. The combination of hydrogen peroxide vapor in close proximity with the article to be sterilized and plasma acts to sterilize the articles, even in closed packages.
Effective sterilization of articles with hydrogen peroxide vapor, either with or without plasma, requires a certain minimum concentration of hydrogen peroxide vapor. If not enough hydrogen peroxide vapor is present, the article will not be sterilized.
If too much hydrogen peroxide is used, the hydrogen peroxide vapor can potentially damage the medical devices, particularly if they contain nylon, neoprene, brass or copper. For hydrogen peroxide absorbent materials, too much peroxide may leave an unacceptable residue on the medical device that may be incompatible with the user or patient. In addition, the use of too much hydrogen peroxide increases the cost of sterilization.
The determination of the appropriate amount of peroxide is especially important for large sterilizers, because the load in large sterilizers can vary more than the load in small sterilizers. One cannot assume the worst-case scenario for each cycle and introduce the maximum possible amount of sterilant. Therefore, there is a need for a method for determining how much hydrogen peroxide is needed to effectively sterilize the medical device without damaging it or leaving too much residual.
There is a need for determining the load of equipment to be sterilized in the sterilization chamber in order to obtain the best efficiency and material compatibility.
One aspect of the present invention relates to a method of determining and sterilizing a load in a sterilization chamber. The method comprises introducing a known amount of sterilant into the sterilization chamber, measuring a parameter related to the concentration of sterilant in the sterilization chamber, where the parameter is measured during or after introducing the sterilant, determining the load in the sterilization chamber from the concentration of sterilant in the sterilization chamber, where the determining is during or after the measuring, repeating the method, if necessary, and sterilizing the load.
Preferably, the known amount of sterilant is less than or equal to 50% of the amount to sterilize a fully loaded chamber. Advantageously, the known amount of sterilant is an amount which produces a pressure in the chamber less than the vapor pressure of the sterilant. Preferably, the sterilant is introduced from a reservoir of sterilant or from a cassette. Advantageously, the sterilant is introduced from at least one cell in the cassette.
In one embodiment, the parameter is measured over a time period, where the time period is the time during the determining. Advantageously, the sterilant comprises a source of hydrogen peroxide. Preferably, the source of peroxide is a liquid solution or a solid peroxide complex.
Advantageously, the concentration of sterilant is measured by a method selected from a spectrophotometric method, measuring the pressure in the sterilization chamber, or measuring with a semiconductor sensor. Preferably, the parameter related to the concentration of sterilant is measured by a spectrophotometric method in the infrared or ultraviolet region.
In a preferred embodiment, the method additionally comprises exposing the load to plasma. Advantageously, the plasma is generated within the chamber. In an alternative embodiment, the method is performed within the chamber and the plasma is generated in a second, separate chamber, and the plasma is flowed into the sterilization chamber.
In one embodiment, the known amount of sterilant is more than the amount of sterilant required to sterilize said load. Preferably, at least a portion of the sterilant is removed with vacuum.
In another embodiment, the determining and sterilizing further comprise aborting the method. Advantageously, aborting the method comprises removing at least a portion of the sterilant with vacuum. Alternatively, aborting the method comprises decomposing at least a portion of the sterilant with plasma.
Advantageously, the load is determined based on the concentration of sterilant. Preferably, the determining is based on the maximum of the parameter related to the concentration of sterilant in the time period. Alternatively, the determining is based on the minimum of the parameter in the time period. Advantageously, the minimum of the parameter in the time period is measured within four minutes after introducing the known amount of sterilant into the chamber.
In another embodiment, the determining is based on the rate of change of the parameter in the time period from the maximum to the minimum. Preferably, the rate of change of the parameter is measured within four minutes from the time when the known amount is introduced into the chamber. Advantageously, the determining is based on the rate of change of the parameter in the time period from the initial time to the maximum in the parameter.